The US Drug Enforcement Agency wants to find out who supplied the actor with a highly-addictive painkiller.

OxyContin – known as Hillbilly Heroin – was found in the Dark Knight star’s body after his death in New York on January 22.

The drug formed part of a lethal cocktail of pills Ledger, 28, accidentally overdosed on. Now DEA agents have revealed that although containers were found at his bedside for every other legally-prescribed drug he took, there was no bottle for the OxyContin.

The opiate-based drug is a favourite on the Hollywood party scene because it can give an instant, long-lasting “high”.

A spokesman for the DEA confirmed: “We are trying to ID the source of the OxyContin.

“If he had a bottle from a friend, was it taken by someone before police and paramedics got to his bedside?” He added: “If necessary witnesses will be subpoenaed to testify.”

The DEA’s stance follows reports last week that Ledger’s friend, actress Mary-Kate Olsen, 22, has refused to be interviewed by agents unless she is guaranteed immunity from prosecution.

Mary-Kate owns the Manhattan loft where Ledger died.

Her attorney Michael Miller said: “She had nothing to do with the drugs found in Heath Ledger’s home or his body and she does not know where he got them.”